The present invention relates to the field of magnetic resonance imaging (MRI) systems and, more particularly, to coils for use in such systems.
It is well known in the field of MRI systems to provide radio frequency signals in the form of circularly polarized or rotating magnetic fields having an axis of rotation aligned with a main magnetic field. It is also well known to use receiving coils to intercept a radio frequency magnetic field generated by a human subject or an object in the presence of the main magnetic field in order to provide an image of the human subject or the object.
Receiving coils of this type were formed as volume coils in order to enclose a volume for receiving a body part such as a leg, arm or hand and intercept the radio magnetic field. See, for example, U.S. Pat. No. 4,411,270 issued to Damadian and U.S. Pat. No. 4,923,459 issued to Nambu. Additionally, surface receiving coils were developed for this purpose. The surface receiving coils were placed adjacent a region of interest. For a surface receiving coil, see U.S. Pat. No. 4,793,356 to Misic et al., for example.
Advances in the field of MRI systems have resulted in modifications to both volume receiving coils and surface receiving coils in order to improve their signal to noise ratios. This was achieved by modifying the receiving coils to receive perpendicular components of the radio frequency magnetic field. These improved coils are known as quadrature coils. Quadrature coils provided a significant signal to noise ratio improvement over non-quadrature coils. See, for example, U.S. Pat. No. 4,467,282 issued to Siebold and U.S. Pat. No. 4,707,664 issued to Fehn.
In U.S. Pat. No. 5,258,717, issued to Misic, a quadrature receiving coil system was provided, along with a data acquisition system. The data acquisition system taught by Misic included multiple image processing channels for processing a plurality of MRI signals and combining the processed signals to produce an image. The receiving coil system of Misic was formed of multiple quadrature receiving coils, the receiving coils being adapted to intercept both of the quadrature components of the magnetic resonance signals in a spatially dependent manner. Such quadrature coil systems provided coverage of a portion of a total target sensitive volume along an axis parallel to the main magnetic field. Consequently, each receiving coil of the system had a sensitive volume smaller than that which would otherwise be necessary. Thus, each receiving coil provided an improved signal to noise ratio for the region within its sensitive volume. Two leads were connected to each receiving coil and each lead was connected to a separate processing channel of the data acquisition system. The outputs of the processing channels were combined and a final data set from the entire target sensitive volume was calculated. The calculated data set had a better signal to noise ratio greater than that which could be achieved with a single receiving coil.
However, the various receiving coils of the prior art described had a number of artifact problems. For example, an image provided using the prior art receiving coils could have artifacts due to aliasing caused when the phase of a signal from a part of the anatomy within the field of the coil duplicates that of a location elsewhere. This occurs because a phase location of 370 degrees appears to the system as a phase location of 10 degrees. Thus, a signal from anatomy at a phase location of −350 or 370 degrees manifests itself in the image at a phase location of 10 degrees within the field of view. Elimination of phase wrap essentially halves the actual phase field of view, shifting from −90 to +90 degrees rather than from −180 to +180 degrees. However, this merely moves the alias location to more than +/−1.5 the field of view rather that eliminating it.
Another form of artifact, sometimes referred to as an annafact, can occur in either the frequency direction or the phase direction within prior art MRI systems. In this type of artifact, an area of anatomy that is at least partially within the excitation field of the body coil has a local Larmour frequency identical to a pixel within the imaging field of view. If there is any excitation and subsequent pickup of this material, it appears within the field of view superimposed upon the desired image, regardless of whether the artifact comes in from the frequency direction or the phase direction. The problems associated with this type of artifact are worsened by the use of higher speed gradients that are shorter in physical size and lower field uniformity.
It is therefore an objective of the invention to provide a coil system and/or method that eliminates soft tissue artifacts and aliasing artifacts typical of prior art systems and/or methods for imaging various regions of interest.
Another objective of the present invention is to provide improved signal to noise performance, for example, by permitting the use of smaller fields of view and thinner slices when performing imaging.
Another objective of the present invention is to provide greater image uniformity than provided in the prior art.
Another objective of the invention is to facilitate complete imaging of the regions of interest during a magnetic resonance imaging (MRI) procedure.